1. Field of the Invention
The present invention relates to a short arc type high voltage electrical discharge electrode, a short arc type high voltage electrical discharge tube, a short arc type high voltage electrical discharge light source apparatus, and their manufacturing methods.
2. Description of Related Art
A high intensity electrical discharge (HID) lamp such as a metal halide lamp, an extra-high voltage mercury lamp and the like are widely used for the light source of a projection type projector such as a liquid crystal projector and the like, and a lighting lamp for a car.
At least a part of an electrical discharge electrode of a metal halide lamp, an extra-high voltage mercury lamp and the like becomes a high temperature, reaching 2000° C. or more during the operation of the electrical discharge electrode.
For this reason, the electrical discharge electrode is usually made of a refractory metal such as tungsten.
As the electrode characteristics required for an electrical discharge lamp, geometry accuracy, the reliability of strength in a high temperature and the like can be cited.
For example, in the metal halide lamp used as a light source of a display device such as a liquid crystal projector TV and the like, and in a light source lamp using a short arc type high voltage electrical discharge tube such as the extra-high voltage mercury lamp and the like, electrical discharge stability as a point light source is especially important.
In this case, changes of an arc spot and the differences of an arc temperature are not preferable because they cause a flicker and the dispersion of lamp luminance.
FIG. 12 is a schematic sectional view of a short arc type high voltage electrical discharge tube in an extra-high voltage mercury lamp, a metal halide lamp and the like.
The electrical discharge tube 100 is composed of a pair of electrical discharge electrodes 103 disposed in an electrical discharge tube body 102 including a sealed hollow 101 at the center thereof. The pair of electrical discharge electrodes 103 is arranged so that their electrical discharge tips may be opposed to each other with a predetermined interval held between them in the sealed hollow 101. Power supply terminals 104 from both the electrodes 103 are airtightly sealed to be derived from both the ends of the electrical discharge tube body 102 onto the outside.
FIG. 13 is a side view of a general electrical discharge electrode 103 of related art. The electrical discharge electrode 103 is composed of an electrode main body 105, which is used as the electrode tip to cause an electrical discharge substantially and is attached to the tip of an electrode center spindle 106 used as an electrification conductor and a mechanical supporter.
FIGS. 14A and 14B are side views of a main body member 105a and a center spindle member 106a constituting the electrode main body 105 and the electrode center spindle 106 of the electrical discharge electrode 103, respectively.
The main body member 105a is made into the shape of a coil of two layers of the form which enlarges the surface area thereof in order to heighten the heat radiation effect of a wire rod including a refractory metal such as tungsten and the like as the principal component, for example.
Moreover, the center spindle member 106a is formed in a cylinder similarly.
The tip portion of the center spindle member 106a is pierced into the center hole of the coil-shaped main body member 105a, and the center spindle member 106a and the main body member 105a at the tip portion are melted by the irradiation of a YAG laser light or the like to form the short arc type high voltage electrical discharge electrode 103 in the shape of a hanging bell having a spherical surface smoothly curved at the tip portion as shown in FIG. 13.
However, this manufacturing method and the structure cannot perform one point irradiation of the laser light because a laser irradiation unit necessary at the time of the melting with the YAG laser has a depth and an extent. Consequently, the focus of the irradiated laser light is easily shifted at each part, and the irradiation power becomes uneven. Thereby, the electrode temperature at the time of electrical discharge operation is dispersed, and changes of the arc spot and the unevenness of the arc temperature arise. Consequently, the optical characteristics of the short arc type high voltage electrical discharge tube composed of the electrical discharge electrodes are dispersed to cause the lowering of the yield.
On the other hand, a proposal of a sintered electrode was made (see Published Japanese Translation of a PCT Application No. 2000-505939). A rod core pin corresponding to the electrode center spindle 106 mentioned above is prepared as the sintered electrode, and the sintered electrode is formed by compressing powder constituting the sintered electrode around the rod core pin and by sintering the powder to be combined with the rod core pin. Thereby the sintered electrode is formed. Alternatively, the sintered electrode is formed by the following steps of: arranging the rod core pin in a compression type mold; injecting a mixture constituting an electrode into the periphery of the rod core pin; and sintering the mixture to combine with the rod core pin.
However, in the case of such a configuration, because a material containing a binder therein is compressed together with the metal powder constituting the sintered electrode around the rod core pin at the time of sintering, the rod core pin is subjected to high temperature processing in the state in which the rod core pin touches the binder. In this case, the embrittlement of the rod core pin by recrystallization of the impurities in the binder is promoted.
Moreover, because the electrode in the state of not being sintered is sintered, the shrinkage ratios of the sintered electrode and the rod core pin are greatly different from each other. For this reason, breakage arises, or an electrical discharge electrode in which distortion remains is constituted.
Owing to such embrittlement, the existence of distortion, and the like, there is the possibility that the rejection rate becomes high, that the uniformity of characteristics is inferior, and that the problems of the durability and the reliability of a heat cycle of high temperature heating at the time of operation and fall of temperature at the time of non-operating arise.
Moreover, it is possible to press the electrode center spindle into the center hole formed in the electrode main body in order to aim at settling the problems mentioned above. But, because the sintered body of tungsten is poor in elasticity, the pressing method cannot be applied to the sintered body.
On the other hand, in a short arc type high voltage electrical discharge electrode, an electrical discharge tube, an electrical discharge light source apparatus and the like, it is desired to further improve luminous efficiency for the further improvement in the reliability of a luminance.
In passing, a metal halide lamp, an extra-high voltage mercury lamp and the like in related art are severally mounted with an electrode similar to the general electrical discharge electrode 103 (see FIG. 13). For example, in order to heighten luminous efficiency, there is generally a method of raising a luminous metal vapor pressure by raising injection power to raise the temperature of the inner wall of an electrical discharge tube. However, a temperature rise of the electrode mainly by ion impact is caused, and it becomes an issue of shortening of life caused by the consumption of the electrode owing to heat and the promotion of the crystallization (the so-called devitrification) of the inner wall of a quartz tube. There is means for thickening the diameter of the electrode center spindle as a measure of the issue. But, if the diameter of the electrode center spindle is thickened, the danger of resulting in an explosion increases owing to the generation of distortion of quartz (SiO2) caused by the difference of the coefficients of thermal expansion between the tungsten W (including tungsten as the principal component) and the quartz of the electrode material at the quartz tube sealing portion.
The inventors of the present invention found that the electrical discharge electrode of related art shown in FIGS. 13, 14A and 14B was bad in the heat conduction from the tip of the electrode main body 105 to the electrical discharge tube body (e.g. a quartz tube body) 102 through the electrode center spindle 106 at the time of the operation of the electrical discharge tube to make it impossible to raise the inner wall temperature of the tube body efficiently, and consequently that the luminous efficiency thereof did not rise, because the electrode main body 105 and the electrode center spindle 106 were connected to each other with voids in a part as shown in the sectional view of FIG. 15.
It is desirable to provide a short arc type high voltage electrical discharge electrode, a short arc type high voltage electrical discharge tube, and/or a short arc type high voltage electrical discharge light source apparatus, each having the stability and the uniformity of characteristics and being excellent in durability and reliability even in the case of adopting an electrode configuration including tungsten as the principal component, and a manufacturing method of each of them.